Rb_0.5V₂O₅, an anhydrous phase of the rubidium intercalate, has been synthesized in a hydrothermal VO(OH)₂–RbCl system. It crystallizes in the monoclinic system C2/m: a= 11.596(2), b= 3.6908(9), c= 9.723(l)Å, β= 100.93(1)° and Z= 4. A single-crystal study (R/R_w= 0.069/0.079) revealed that the V₂O₅ layers are isostructural with those of K_0.5V₂O₅ in Part 4 which consist of zigzag chains of edge-sharing VO₆ octahedra. The interlayer Rb atom forms an RbO₈ rectangular prism with the apical oxygens of the VO₆ octahedra in contrast to the KO₇ coordination in K_0.5 V₂O₅˙Rb_0.5V₂O₅ with 9.55 Å spacing was oxidized by hydrogen peroxide to a hydrated phase, Rb_0.4V₂O₅˙0.8H₂O with 10.85 Å spacing. This compound was reduced by rubidium iodide to a less hydrated phase, Rb_0.4V₂O₅·0.5H₂O with 10.41 Å spacing, rather than to Rb_0.5V₂O₅. Interlayer sites for monovalent cations in anhydrous phases are suggested to be correlated with cationic sizes and contents.